Planarian lifespan and age in human years
Average lifespan Biologically immortal (can revert to youth)
Planarians that reproduce by splitting in two keep renewing their bodies with stem cells, so in theory they may never die of old age.
Sources
How we convert ages: method.
More Microscopic life
Roundworm (C. elegans)Lifespan about 21 days
Tardigrade (water bear)Lifespan about 12 months
Water flea (Daphnia)Lifespan about 20 days
Moina (water flea)Lifespan about 12 days
Cyclopoid copepodLifespan about 1 months
Rotifer (Brachionus plicatilis)Lifespan about 4 days
Bdelloid rotiferLifespan about 14 days
Brine shrimp (Artemia)Lifespan about 3 months
HydraLifespan Biologically immortal (can revert to youth)